Agonist RS Research Articles

The dopamine uptake inhibitor, WIN 35,428, has sigma2 receptor agonist effects that contribute to its self‐administration (848.10)

Sigma receptors (σRs) are intracellular chaperone proteins implicated in several diseases, as well as psychiatric disorders and drug abuse. Several in vivo studies show blockade with σR antagonists of several cocaine effects, though not its self administration (SA). The present study assessed whether that insensitivity was specific to cocaine SA using the dopamine uptake inhibitors (WIN35,428, methylphenidate, nomifensine) in rats trained with cocaine SA. As with cocaine, σR antagonists (BD 1008, BD 1047, BD 1063, SN 79, SN 167, (±)SM 21, CM 304, CM 398) were ineffective against methylphenidate and nomifensine SA across a range of doses that antagonized SA of σR agonists (DTG, PRE 084, (+)pentazocine). In contrast, all antagonists tested, except CM 304 produced dose dependent insurmountable antagonism of WIN35,428 SA. CM 304 was equipotent in blocking the SA of DTG, (+)‐pentazocine, and PRE‐084, but only marginally potent in blocking SA of WIN35,428. The antagonists SN 79, SN 167, CM 398, and (±)SM 21 were more potent in blocking DTG than either PRE 084 or (+)pentazocine SA. These antagonists also blocked WIN35,428 SA, though three fold less potently than against DTG. The results are consistent with σR radioligand binding data for WIN35,428, the nonselective σ1/2R agonist effects of DTG, selective σ1R antagonist effects of CM 304, and that WIN 35,428 has σ2R agonist effects that contribute to its self administration. Char 1219

Anxiolytic-like effects observed in rats exposed to the elevated zero-maze following treatment with 5-HT2/5-HT3/5-HT4 ligands.

The present study examined the effects of administering selective 5-HT antagonists and agonists to rats tested in the elevated zero-maze (EZM) model of anxiety. The EZM paradigm has advantages over the elevated plus-maze (EPM) paradigm with respect to measuring anxiety, yet has been utilized less frequently. Three experiments were conducted each with a diazepam control (0.25, 0.5 and 0.75 mg/kg). In the first experiment, we administered the 5-HT2C antagonist RS 102221 (0.5, 1.0, and 2.0 mg/kg) and 5-HT2C agonist MK-212 (0.25, 0.5 and 0.75 mg/kg); in the second experiment, we administered the 5-HT3 antagonist Y-25130 (0.1, 1.0 and 3.0 mg/kg) and 5-HT3 agonist SR 57227A (0.1, 1.0 and 3.0 mg/kg), and in the third experiment, we administered the 5-HT4 antagonist RS 39604 (0.01, 0.1, 1.0 mg/kg) and 5-HT4 agonist RS 67333 (0.01, 0.1 and 0.5 mg/kg). The administration of 5-HT2/3/4 subtype antagonists all generated behavioral profiles indicative of anxiolytic-like effects in the EZM, which was apparent from examination of both traditional and ethological measures. While little effect was observed from 5-HT2 and 5-HT3 agonists, the 5-HT4 agonist RS 67333 was found to produce a paradoxical anxiolytic-like effect similar to that produced by the 5-HT4 antagonist RS 39604. We conclude by discussing the implications of these findings.

Role of the σRs for Development of Medications.

Role of the σRs for Development of Medications.

Fluvoxamine rescues mitochondrial Ca2 + transport and ATP production through σ1-receptor in hypertrophic cardiomyocytes

AimsWe previously reported that fluvoxamine, a selective serotonin reuptake inhibitor with high affinity for the σ1-receptor (σ1R), ameliorates cardiac hypertrophy and dysfunction via σ1R stimulation. Although σ1R on non-cardiomyocytes interacts with the IP3 receptor (IP3R) to promote mitochondrial Ca2+ transport, little is known about its physiological and pathological relevance in cardiomyocytes. Main methodsHere we performed Ca2+ imaging and measured ATP production to define the role of σ1Rs in regulating sarcoplasmic reticulum (SR)-mitochondrial Ca2+ transport in neonatal rat ventricular cardiomyocytes treated with angiotensin II to promote hypertrophy. Key findingThese cardiomyocytes exhibited imbalances in expression levels of σ1R and IP3R and impairments in both phenylephrine-induced mitochondrial Ca2+ mobilization from the SR and ATP production. Interestingly, σ1R stimulation with fluvoxamine rescued impaired mitochondrial Ca2+ mobilization and ATP production, an effect abolished by treatment of cells with the σ1R antagonist, NE-100. Under physiological conditions, fluvoxamine stimulation of σ1Rs suppressed intracellular Ca2+ mobilization through IP3Rs and ryanodine receptors (RyRs). In vivo, chronic administration of fluvoxamine to TAC mice also rescued impaired ATP production. SignificanceThese results suggest that σ1R stimulation with fluvoxamine promotes SR-mitochondrial Ca2+ transport and mitochondrial ATP production, whereas σ1R stimulation suppresses intracellular Ca2+ overload through IP3Rs and RyRs. These mechanisms likely underlie in part the anti-hypertrophic and cardioprotective action of the σ1R agonists including fluvoxamine.

Stimulants as Specific Inducers of Dopamine-Independent σ Agonist Self-Administration in Rats

A previous study showed that cocaine self-administration induced dopamine-independent reinforcing effects of σ agonists mediated by their selective actions at σ1 receptors (σ1Rs), which are intracellularly mobile chaperone proteins implicated in abuse-related effects of stimulants. The present study assessed whether the induction was specific to self-administration of cocaine. Rats were trained to self-administer the dopamine releaser, d-methamphetamine (0.01-0.32 mg/kg per injection), the μ-opioid receptor agonist, heroin (0.001-0.032 mg/kg per injection), and the noncompetitive N-methyl-d-aspartate receptor/channel antagonist ketamine (0.032-1.0 mg/kg per injection). As with cocaine, self-administration of d-methamphetamine induced reinforcing effects of the selective σ1R agonists PRE-084 [2-(4-morpholinethyl)1-phenylcyclohexanecarboxylate hydrochloride] and (+)-pentazocine (0.032-1.0 mg/kg per injection, each). In contrast, neither self-administration of heroin nor ketamine induced PRE-084 or (+)-pentazocine (0.032-10 mg/kg per injection, each) self-administration. Although the σ1R agonists did not maintain responding in subjects with histories of heroin or ketamine self-administration, substitution for those drugs was obtained with appropriate agonists (e.g., remifentanil, 0.1-3.2 µg/kg per injection, for heroin and (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine ((+)-MK 801; dizocilpine), 0.32-10.0 µg/kg per injection, for ketamine). The σR antagonist N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine dihydrobromide (BD 1008; 1.0-10 mg/kg) dose-dependently blocked PRE-084 self-administration but was inactive against d-methamphetamine, heroin, and ketamine. In contrast, PRE-084 self-administration was affected neither by the dopamine receptor antagonist (+)-butaclamol (10-100 μg/kg) nor by the opioid antagonist (-)-naltrexone (1.0-10 mg/kg), whereas these antagonists were active against d-methamphetamine and heroin self-administration, respectively. The results indicate that experience specifically with indirect-acting dopamine agonists induces reinforcing effects of previously inactive σ1R agonists. It is further suggested that induced σ1R reinforcing mechanisms may play an essential role in treatment-resistant stimulant abuse, suggesting new approaches for the development of effective medications for its treatment.

Specificity of cocaine‐induced dopamine‐independent sigma agonist self‐administration

Sigma1 receptors (σ1Rs) are intracellularly‐mobile chaperone proteins implicated in several diseases, as well as psychiatric disorders and substance abuse. A previous study showed that cocaine self‐administration (SA) induced dopamine (DA)‐independent reinforcing effects of selective σ1R agonists. The present study assessed whether the induction was specific to SA of cocaine. Selective σ1R agonists (PRE‐084, (+)‐pentazocine) lacked reinforcing effects in drug‐naive rats. SA was not obtained with 28 experimental sessions with the opportunity to selfadminister either σ1R agonist. The same exposure to cocaine was sufficient for its SA, after which robust SA of σ1R agonists was obtained. As with cocaine, SA of the indirect‐acting DA receptor agonist d‐methamphetamine induced the reinforcing effects of PRE‐084. In contrast, experiences with food reinforcement, or SA of (−)‐heroin or (±)‐ketamine were ineffective as inducers of PRE‐084 reinforcement. The results indicate that experience with indirect‐acting DA receptor agonists induces reinforcing effects of previously inactive σ1R agonists whereas reinforcement through different modalities is ineffective. It is further suggested that induced σ1R mechanisms may play an essential role in treatment‐resistant stimulant abuse, suggesting new approaches for the development of effective medications for stimulant abuse. Supported by NIDA IRP.

Stimulation of σ1-receptor restores abnormal mitochondrial Ca2 + mobilization and ATP production following cardiac hypertrophy

BackgroundWe previously reported that the σ1-receptor (σ1R) is down-regulated following cardiac hypertrophy and dysfunction in transverse aortic constriction (TAC) mice. Here we address how σ1R stimulation with the selective σ1R agonist SA4503 restores hypertrophy-induced cardiac dysfunction through σ1R localized in the sarcoplasmic reticulum (SR). MethodsWe first confirmed anti-hypertrophic effects of SA4503 (0.1–1μM) in cultured cardiomyocytes exposed to angiotensin II (Ang II). Then, to confirm the ameliorative effects of σ1R stimulation in vivo, we administered SA4503 (1.0mg/kg) and the σ1R antagonist NE-100 (1.0mg/kg) orally to TAC mice for 4weeks (once daily). Resultsσ1R stimulation with SA4503 significantly inhibited Ang II-induced cardiomyocyte hypertrophy. Ang II exposure for 72h impaired phenylephrine (PE)-induced Ca2+ mobilization from the SR into both the cytosol and mitochondria. Treatment of cardiomyocytes with SA4503 largely restored PE-induced Ca2+ mobilization into mitochondria. Exposure of cardiomyocytes to Ang II for 72h decreased basal ATP content and PE-induced ATP production concomitant with reduced mitochondrial size, while SA4503 treatment completely restored ATP production and mitochondrial size. Pretreatment with NE-100 or siRNA abolished these effects. Chronic SA4503 administration also significantly attenuated myocardial hypertrophy and restored ATP production in TAC mice. SA4503 administration also decreased hypertrophy-induced impairments in LV contractile function. Conclusionsσ1R stimulation with the specific agonist SA4503 ameliorates cardiac hypertrophy and dysfunction by restoring both mitochondrial Ca2+ mobilization and ATP production via σ1R stimulation. General significanceOur observations suggest that σ1R stimulation represents a new therapeutic strategy to rescue the heart from hypertrophic dysfunction.

Vascular Endothelial σ<sub>1</sub>-Receptor Stimulation With SA4503 Rescues Aortic Relaxation via Akt/eNOS Signaling in Ovariectomized Rats With Aortic Banding

We previously reported that σ1-receptor (σ1R) expression in the thoracic aorta decreased after pressure overload (PO) induced by abdominal aortic banding in ovariectomized (OVX) rats. Here, we asked whether stimulation of σ1R with the selective agonist SA4503 elicits functional recovery of aortic vasodilation and constriction following vascular injury in OVX rats with PO. SA4503 (0.3-1.0mg/kg) and NE-100 (a σ1R antagonist, 1.0mg/kg) were administered orally for 4 weeks (once daily) to OVX-PO rats. Vascular functions of isolated descending aorta were measured following phenylephrine (PE)- or endothelin-1 (ET-1)-induced vasoconstriction and acetylcholine (ACh)- or clonidine-induced vasodilation. SA4503 administration rescued PO-induced σ1R decreases in aortic smooth muscle and endothelial cells. SA4503 treatment also rescued PO-induced impairments in ACh- and clonidine-induced vasodilation without affecting PE- and ET-1-induced vasoconstriction. Ameliorated ACh- and clonidine-induced vasodilation was closely associated with increased Akt activity and in turn endothelial nitric oxide synthase (eNOS) phosphorylation. The SA4503-mediated improvement of vasodilation was blocked by NE-100 treatment. σ1R is downregulated following PO-induced endothelial injury in OVX rats. The selective σ1R agonist SA4503 rescues impaired endothelium-dependent vasodilation in the aorta from OVX-PO rats through σ1R stimulation, enhancing eNOS-cGMP signaling in vascular endothelial cells. These observations encourage development of novel therapeutics targeting σ1R to prevent vascular endothelial injury in vascular diseases.

Sigma-1 Receptor Expression in Sensory Neurons and the Effect of Painful Peripheral Nerve Injury

BackgroundThe sigma-1 receptor (σ1R), an endoplasmic reticulum chaperone protein, is widely distributed and regulates numerous intracellular processes in neurons. Nerve injury alters the structure and function of axotomized dorsal root ganglion (DRG) neurons, contributing to the development of pain. The σ1R is enriched in the spinal cord and modulates pain after peripheral nerve injury. However, σ1R expression in the DRG has not been studied. We therefore characterized σ1R expression in DRGs at baseline and following spinal nerve ligation (SNL) in rats.ResultsImmunohistochemical (IHC) studies in DRG sections show σ1R in both neuronal somata and satellite glial cells. The punctate distribution of σ1R in the neuronal cytoplasm suggests expression in the endoplasmic reticulum. When classified by neuronal size, large neurons (>1300 μm) showed higher levels of σ1R staining than other groups (700-1300 μm, <700 μm). Comparing σ1R expression in neuronal groups characterized by expression of calcitonin gene-related peptide (CGRP), isolectin-B4 (IB4) and neurofilament-200 (NF-200), we found σ1R expression in all three neuronal subpopulations, with highest levels of σ1R expression in the NF-200 group. After SNL, lysates from L5 DRGs that contains axotomized neurons showed decreased σ1R protein but unaffected transcript level, compared with Control DRGs. IHC images also showed decreased σ1R protein expression, in SNL L5 DRGs, and to a lesser extent in the neighboring SNL L4 DRGs. Neurons labeled by CGRP and NF-200 showed decreased σ1R expression in L5 and, to a lesser extent, L4 DRGs. In IB4-labeled neurons, σ1R expression decreased only in axotomized L5 DRGs. Satellite cells also showed decreased σ1R expression in L5 DRGs after SNL.ConclusionsOur data show that σ1R is present in both sensory neurons and satellite cells in rat DRGs. Expression of σ1R is down-regulated in axotomized neurons as well as in their accompanying satellite glial cells, while neighboring uninjured neurons show a lesser down-regulation. Therefore, elevated σ1R expression in neuropathic pain is not an explanation for pain relief after σ1R blockade. This implies that increased levels of endogenous σ1R agonists may play a role, and diminished neuroprotection from loss of glial σ1R may be a contributing factor.

Early administration of RS 67333, a specific 5-HT4 receptor agonist, prevents amyloidogenesis and behavioral deficits in the 5XFAD mouse model of Alzheimer’s disease

Amyloid β (Aβ) accumulation is considered the main culprit in the pathogenesis of Alzheimer’s disease (AD). Recent studies suggest that decreasing Aβ production at very early stages of AD could be a promising strategy to slow down disease progression. Serotonin 5-HT4 receptor activation stimulates α-cleavage of the amyloid precursor protein (APP), leading to the release of the soluble and neurotrophic sAPPα fragment and thus precluding Aβ formation. Using the 5XFAD mouse model of AD that shows accelerated Aβ deposition, we investigated the effect of chronic treatments (treatment onset at different ages and different durations) with the 5-HT4 receptor agonist RS 67333 during the asymptomatic phase of the disease. Chronic administration of RS 67333 decreased concomitantly the number of amyloid plaques and the level of Aβ species. Reduction of Aβ levels was accompanied by a striking decrease in hippocampal astrogliosis and microgliosis. RS 67333 also transiently increased sAPPα concentration in the cerebrospinal fluid and brain. Moreover, a specific 5-HT4 receptor antagonist (RS 39604) prevented the RS 67333-mediated reduction of the amyloid pathology. Finally, the novel object recognition test deficits of 5XFAD mice were reversed by chronic treatment with RS 67333. Collectively, these results strongly highlight this 5-HT4 receptor agonist as a promising disease modifying-agent for AD.

5-HT4 Receptor Stimulation Leads to Soluble AβPPα Production Through MMP-9 Upregulation

Serotonin 4 (5-HT4) receptor signaling does not only have the physiological function of improving cognition, but might also be helpful in the therapy of Alzheimer's disease (AD) through regulation of the production of soluble amyloid-β protein precursor alpha (sAβPPα). To analyze the relationship between 5-HT4 receptor signaling and sAβPPα production, we stably transfected H4 cells with AβPP and 5-HT4 receptor (H4/AβPP/5-HT4 cells). We found that 24-h incubation with the 5-HT4 receptor agonist RS-67333 upregulates matrix metalloproteinase-9 (MMP-9). Furthermore, MMP-9 overexpression enhanced sAβPPα levels, whereas knockdown with MMP-9 siRNA decreased sAβPPα levels. When RS-67333 was injected for 10 days in Tg2576 mice, a model of amyloid-β peptide (Aβ) deposition, there was an increase in hippocampal levels of sAβPPα, C-terminal fragment α, and MMP-9, as well as a decrease in hippocampal senile plaque number and levels of the 40 amino acid peptide, Aβ40. Taken all together, these experiments demonstrate that 5-HT4 receptor stimulation induces expression of MMP-9 which cleaves AβPP through α-secretase-like activity, leading to an increase of sAβPPα levels and a reduction of Aβ load.

1083 The Highly Selective Sigma-1 Receptor Agonist Pre-084 Reduces Inflammation-Sensitized Hyperoxia-Induced Injury in the Developing Rat Brain

BackgroundSupraphysiologic oxygen concentrations are toxic to the developing brain. Inflammatory processes increase the risk of brain injury. We have previously shown a protective effect of dextromethorphan, a NMDA receptor antagonist...

299 PRE-084, A Sigma-1 Receptor Ligand, Protects Against Excitotoxic Perinatal Brain Injury in Newborn Mice

BackgroundExcessive glutamate release followed by N-methyl-D-aspartate receptor (NMDAR) activation displays an important cascade in the pathophysiology of perinatal brain injury. We have previously shown that dextromethorphan, a low-affinity NMDAR antagonist,...

A novel regulator of angiogenesis in endothelial cells: 5-hydroxytriptamine 4 receptor

The 5-hydroxytryptamine type 4 receptor (5-HT(4)R) regulates many physiological processes, including learning and memory, cognition, and gastrointestinal motility. Little is known about its role in angiogenesis. Using mouse hindlimb ischemia model of angiogenesis, we observed a significant reduction of limb blood flow recovery 14 days after ischemia and a decrease in density of CD31-positive vessels in adductor muscles in 5-HT(4)R(-/-) mice compared to wild type littermates. Our in vitro data indicated that 5-HT(4)R endogenously expressed in endothelial cells (ECs) may promote angiogenesis. Inhibition of the receptor with 5-HT(4)R antagonist RS 39604 reduced EC capillary tube formation in the reconstituted basement membrane. Using Boyden chamber migration assay and wound healing "scratch" assay, we demonstrated that RS 39604 treatment significantly suppressed EC migration. Transendothelial resistance measurement and immunofluorescence analysis showed that a 5-HT(4)R agonist RS 67333 led to an increase in endothelial permeability, actin stress fiber and interendothelial gap formation. Importantly, we provided the evidence that 5-HT(4)R-regulated EC migration may be mediated by Gα13 and RhoA. Our results suggest a prominent role of 5-HT(4)R in promoting angiogenesis and identify 5-HT(4)R as a potential therapeutic target for modulating angiogenesis under pathological conditions.

Anti-amnesic effect of neurosteroid PREGS in Aβ25–35-injected mice through σ1 receptor- and α7nAChR-mediated neuroprotection

A single intracerebroventricular injection of β-amyloid 25–35 peptide (Aβ25–35) (9 nmol/mouse) induces the spatial cognitive deterioration and approximately 50% loss of pyramidal cells in hippocampal CA1 region within 1 week. The present study focused on exploring the effects of neurosteroid pregnenolone sulfate (PREGS), in comparison with the selective agonists of sigma-1 receptor (σ1R) and α7 nicotinic acetylcholine receptor (α7nAChR), on the cognitive deficits and the death of pyramidal cells in Aβ25–35-mice. Herein, we reported that the administration of PREGS (1–100 mg/kg) for 7 days after Aβ25–35-injection could dose-dependently ameliorate the cognitive deficits and attenuate the apoptosis of pyramidal cells. Either the σ1R antagonist NE100 or the α7nAChR antagonist MLA could block the neuroprotection of PREGS in Aβ25–35-mice. Both the σ1R agonist PRE084 and the α7nAChR agonist DMXB could mimic the PREGS-neuroprotection against the Aβ25–35-neurotoxicity. The neuroprotection of PRE084 was attenuated by MLA, but the DMXB-action was insensitive to NE100. The neuroprotection of PREGS, PRE084 or DMXB was blocked by the phosphatidylinositol-3-kinase (PI3K) inhibitor LY294002, whereas only the effect of PREGS or PRE084 was sensitive to the MAPK/ERK kinase (MEK) inhibitor U0126. PREGS prevented Aβ25–35-inhibited Akt (Serine/threonine kinase) phosphorylation leading to increase in caspase-3 activity, which was σ1R- and α7nAChR-dependent. By contrast, PREGS-rescued reduction of extracellular signal-related kinase-2 (ERK2) phosphorylation in Aβ25–35-mice only required the activation of σ1R. Blockage of PREGS-neuroprotection by LY294002 significantly attenuated its anti-amnesic effect in Aβ25–35-mice. The findings indicate that the anti-amnesic effects of PREGS in Aβ25–35-mice depend on the σ1R- and α7nAChR-mediated neuroprotection.

Neuroprotective effects of the sigma-1 receptor ligand PRE-084 against excitotoxic perinatal brain injury in newborn mice

Excessive glutamate release followed by N-methyl-d-aspartate receptor (NMDAR) activation plays a crucial role in perinatal brain injury. We have previously shown that dextromethorphan, a low‐affinity NMDAR antagonist with anti-inflammatory properties, is neuroprotective against neonatal excitotoxic brain injury. Of interest, dextromethorphan is also a sigma-1 receptor (σ1R) agonist. The pharmacologic class of σ1R agonists has yielded propitious results in various animal models of adult central nervous system pathology. In an established neonatal mouse model of excitotoxic brain injury, we evaluated the effect of the selective σ1R agonist 2-(4-morpholinethyl) 1-phenylcyclohexanecarboxylate (PRE-084). A single intraperitoneal injection of 0.1μg/g (low dose) or 10μg/g (high dose) bodyweight (bw) PRE-084, given 1h after the excitotoxic insult, significantly reduced lesion size in cortical gray matter 24h and 120h after the insult. Repetitive injections of 0.1μg/g PRE-084 proved to be equally effective. PRE-084 treatment resulted in a decrease in cell death indicated by reduced TUNEL positivity and caspase-3 activation. Furthermore, it lowered the number of isolectin B4-positive, activated microglial cells. PRE‐084 had no effect on developmental apoptosis in the undamaged brain. In vitro findings in primary hippocampal neurons suggest that PRE-084 treatment provides partial protection against glutamate induced morphological and functional changes. For excitotoxicity as playing a crucial role in the pathogenesis of perinatal brain injury, we demonstrate for the first time that systemic treatment with the highly selective σ1R agonist PRE-084 protects against NMDAR-mediated excitotoxic brain damage.

A Biphasic and Brain-Region Selective Down-Regulation of Cyclic Adenosine Monophosphate Concentrations Supports Object Recognition in the Rat

BackgroundWe aimed to further understand the relationship between cAMP concentration and mnesic performance.Methods and FindingsRats were injected with milrinone (PDE3 inhibitor, 0.3 mg/kg, i.p.), rolipram (PDE4 inhibitor, 0.3 mg/kg, i.p.) and/or the selective 5-HT4R agonist RS 67333 (1 mg/kg, i.p.) before testing in the object recognition paradigm. Cyclic AMP concentrations were measured in brain structures linked to episodic-like memory (i.e. hippocampus, prefrontal and perirhinal cortices) before or after either the sample or the testing phase. Except in the hippocampus of rolipram treated-rats, all treatment increased cAMP levels in each brain sub-region studied before the sample phase. After the sample phase, cAMP levels were significantly increased in hippocampus (1.8 fold), prefrontal (1.3 fold) and perirhinal (1.3 fold) cortices from controls rat while decreased in prefrontal cortex (∼0.83 to 0.62 fold) from drug-treated rats (except for milrinone+RS 67333 treatment). After the testing phase, cAMP concentrations were still increased in both the hippocampus (2.76 fold) and the perirhinal cortex (2.1 fold) from controls animals. Minor increase were reported in hippocampus and perirhinal cortex from both rolipram (respectively, 1.44 fold and 1.70 fold) and milrinone (respectively 1.46 fold and 1.56 fold)-treated rat. Following the paradigm, cAMP levels were significantly lower in the hippocampus, prefrontal and perirhinal cortices from drug-treated rat when compared to controls animals, however, only drug-treated rats spent longer time exploring the novel object during the testing phase (inter-phase interval of 4 h).ConclusionsOur results strongly suggest that a “pre-sample” early increase in cAMP levels followed by a specific lowering of cAMP concentrations in each brain sub-region linked to the object recognition paradigm support learning efficacy after a middle-term delay.

Lack of Effect of the 5-HT4 Receptor Ligands RS 67333 and RS 39604 on Murine Agonistic Behaviour

In comparison to studies investigating the roles of 5-HT1, 5-HT2 and 5-HT3 receptors in aggressive behaviour there is a dearth of material examining the function of 5-HT4 receptors in this behaviour. In view of this, the current study examined the effects of the 5-HT4 receptor partial agonist RS 67333 and antagonist RS 39604 in murine agonistic behaviour. RS 67333 failed to produce any significant changes in the offensive. Significant variation in the frequency of evade behaviour was detected but this occurred between treatment groups rather than with controls. Interestingly, both the frequency and duration of stretched attend behaviour were increased by RS 67333 0.1 mg/kg, a result indicative of increased risk assessment. The administration of RS 39604 (0.01 - 1 mg/kg) produced significant variation in the fre-quency and duration of following, and aggressive grooming. Frozen crouch behaviour was also increased significantly at 0.1 mg/kg. It is concluded that since the 5-HT4 receptor ligands employed in this study produced very few significant behavioural effects across the treatment groups, 5-HT4 receptors do not play a role in the modulation of murine aggressive behaviour.

Lack of cocaine-like discriminative-stimulus effects of σ-receptor agonists in rats

Previous studies demonstrated the effectiveness of selective σ-receptor (σR) agonists [1,3-di-o-tolylguanidine (DTG), PRE-084] as reinforcers in rats trained to self-administer cocaine. Similar to cocaine, these drugs increased nucleus accumbens shell dopamine levels, and effects of DTG, but not PRE-084, on dopamine seemed to be mediated by σRs. In addition, σR antagonists blocked self-administration of σR agonists, but were inactive against reinforcing and neurochemical effects of cocaine. Thus, pharmacologically distinct mechanisms likely underlie the reinforcing and neurochemical effects of σR agonists and cocaine. This study further examined the cocaine-like effects of σR agonists in rats trained to discriminate injections of cocaine from saline to assess the similarity of their subjective effects. Standard dopamine-uptake inhibitors (WIN 35,428, methylphenidate), but neither σR agonist (PRE-084, DTG), produced full cocaine-like discriminative-stimulus effects. The lack of effects of σR agonists was obtained regardless of route of administration (intraperitoneal, subcutaneous, or intravenous) or pretreatment time (5 or 30 min before sessions). The present results demonstrate differences in the discriminative-stimulus effects of cocaine and selective σR agonists, indicating that an overlap of subjective effects is not necessary for σR agonist self-administration. The previously found differences in neurochemical effects of cocaine and σR agonists may contribute to their different subjective effects.

A Role for Sigma Receptors in Stimulant Self Administration and Addiction

Sigma1 receptors (σ1Rs) represent a structurally unique class of intracellular proteins that function as chaperones. σ1Rs translocate from the mitochondria-associated membrane to the cell nucleus or cell membrane, and through protein-protein interactions influence several targets, including ion channels, G-protein-coupled receptors, lipids, and other signaling proteins. Several studies have demonstrated that σR antagonists block stimulant-induced behavioral effects, including ambulatory activity, sensitization, and acute toxicities. Curiously, the effects of stimulants have been blocked by σR antagonists tested under place-conditioning but not self-administration procedures, indicating fundamental differences in the mechanisms underlying these two effects. The self administration of σR agonists has been found in subjects previously trained to self administer cocaine. The reinforcing effects of the σR agonists were blocked by σR antagonists. Additionally, σR agonists were found to increase dopamine concentrations in the nucleus accumbens shell, a brain region considered important for the reinforcing effects of abused drugs. Although the effects of the σR agonist, DTG, on dopamine were obtained at doses that approximated those that maintained self administration behavior those of another agonist, PRE-084 required higher doses. The effects of DTG were antagonized by non-selective or a preferential σ2R antagonist but not by a preferential σ1R antagonist. The effects of PRE-084 on dopamine were insensitive to σR antagonists. The data suggest that the self administration of σR agonists is independent of dopamine and the findings are discussed in light of a hypothesis that cocaine has both intracellular actions mediated by σRs, as well as extracellular actions mediated through conventionally studied mechanisms. The co-activation and potential interactions among these mechanisms, in particular those involving the intracellular chaperone σRs, may lead to the pernicious addictive effects of stimulant drugs.